How 'Atmospheric Rivers' Are Bringing Rain And Snow To California05:51

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Raindrops appear on a window as a man crosses a street in San Francisco, Wednesday, Jan. 4, 2017. (Jeff Chiu/AP)

California is getting some wet weather this week, and it's all because of a phenomenon called Atmospheric Rivers — long, relatively narrow regions in the atmosphere that transport water vapor, which is then released as rain or snow.

Marty Ralph, a research meteorologist and the director of the Center for Western Weather and Water Extremes at Scripps Institution of Oceanography, tells Here & Now's Meghna Chakrabarti that these "rivers in the sky" are essential to California's water supply — but can also lead to major flooding.